This invention relates to a method of processing a glass substrate for a magnetic disk to a desired degree of roughness and also to a suspension with abrasive particles for such processing.
A substrate for a magnetic disk has its surface mirror-polished and then subjected to a texturing process such that the magnetic disk will be provided with magnetic non-isotropy to improve its magnetic characteristic as a magnetic memory medium and that the adsorption between the magnetic head and the surface of the magnetic disk can be prevented when the hard disk drive is not operating. A magnetic layer is formed thereabove, say, by sputtering. As a substrate for such a magnetic disk, use is usually made of a so-called aluminum substrate with a disk made of an aluminum and plated with Nixe2x80x94P. Such an aluminum substrate is mirror-polished, as disclosed in Japanese Patent Publication Tokkai 8-96355, by sandwiching it between urethane foam pads impregnated with free abrasive particles while a polishing liquid such as an aqueous solution of a surfactant is supplied, and a texturing process is thereafter carried out by pressing a polishing tape of a non-woven cloth material or the like against it while a suspension having abrasive particles suspended in a dispersing liquid mainly of water is supplied.
Glass substrates are recently coming to be used for producing magnetic disks. A glass substrate, too, is required to have its surface processed to be smooth and flat and textured. The surface of a glass surface, however, is different from that of an aluminum substrate comprising an Nixe2x80x94P plated membrane, being hard and fragile. If the same process used on aluminum substrates were carried out on a glass substrate, therefore, there would be formed unwanted protrusions and irregular scratches and it is not possible to accurately obtain a surface with desired roughness (including a smooth and flat surface). In particular, there is the problem that uniform and fine textured lines cannot be formed.
It is therefore an object of this invention to provide a method by which one or both of the surfaces of a glass substrate for a magnetic disk can be processed to a desired level of roughness and in particular uniform and fine textured lines can be formed thereon without any unwanted protrusions or irregular scratches, as well as a suspension with abrasive particles which may be used for such a process.
It is also an object of this invention to provide glass substrates for a magnetic disk with one or both surfaces processed by such a method.
A method embodying this invention for surface-processing a glass substrate for a magnetic disk, with which the above and other objects can be accomplished, may be characterized as comprising the steps of rotating the glass substrate, and causing an abrasive tape selected according to the purpose of surface polishing to be pressed and to run against it, while supplying a suspension with abrasive particles to its surface.
A suspension with abrasive particles embodying this invention may be characterized as comprising abrasive particles having diameters selected according to the purpose of the processing and an aqueous solution containing a reacting liquid capable of causing a solid phase reaction on the contacting surface between these abrasive particles and the surface of a glass substrate.
As described above, a glass substrate for a magnetic disk has its surface of surfaces polished according to this invention by rotating it and causing an abrasive tape selected according to the purpose of the surface polishing to be pressed and to run against it, while supplying a suspension which contains abrasive particles and a reacting liquid to the surface being abraded. The average diameter of the abrasive particles in the suspension is determined according to the purpose of the surface processing. As for the reacting liquid capable of causing a solid phase reaction on the contacting surface between these abrasive particles and the surface of the glass substrate, an aqueous solution containing hydroxyl group and having etching capability on a glass material is used such as solutions containing ammonium salts such as tetramethyl ammonium hydroxide. Neutral and alkaline suspensions may be used but neutral ones are preferable in view of environmental problems associated with the disposal of used liquids.
When a suspension with abrasive particles according to this invention is applied to the surface of a glass substrate, a solid phase reaction takes place on the contact boundary surfaces between the abrasive particles and the glass substrate surface and a heterogeneous substance such as a glass hydrate is generated chemically. The glass hydrate thus generated is mechanically removed from the substrate surface by the abrasive particles during the course of the polishing process. Thus, the substrate surface is polished according to this invention by a chemical-mechanical polishing process and hence the polishing is effected in units of very small areas, or the target surface is polished extremely finely. The method is advantageous also because it relies upon a chemical reaction and hence the deterioration due to the processing is negligibly small.
Polishing tapes capable of collecting polished-off debris generated during the polishing process into its interior are preferably to be used. Examples of such a polishing tape include polishing tapes with a layer of foam material on the surface of a plastic tape, woven and non-woven cloth tapes comprising plastic fibers and planted tapes having plastic fibers planted into the surface of a plastic tape. In the case of a tape with a layer of foam material, debris particles are collected into the foams exposed on the surface of the layer of foam material. In the case of a woven or non-woven cloth tape or a planted tape, they are taken into the space between the fibers and removed from the surface of the glass substrate as trapped inside the polishing tape.
There are two objectives to the polishing, one being for flattening so as to improve the smoothness and evenness of the surface and the other being the so-called texturing process for forming concentric textured lines on the surface. In the former, the surface roughness is reduced by the polishing process but since the surface roughness cannot be reduced to zero as a practical matter, the kind of smooth and flat surfaces obtainable by a process embodying this invention will also be referred to as a xe2x80x9crough surfacexe2x80x9d.
The surface roughness of such glass substrates is controlled by a combination of the type of polishing tape and the diameters of the abrasive particles in the suspension. For example, if a woven or non-woven tape or a planted tape comprising plastic fibers is used as the polishing tape, textured lines without unwanted protrusions or irregular scratches can be formed. If abrasive particles with relatively large diameters are used in such a case, the surface roughness becomes larger. If abrasive particles with relatively small diameters are used, the surface roughness becomes smaller. A flat surface processing can be effected by using a polishing tape of the kind having an elastic layer of a foam material on the surface of a plastic tape. As in the case of a texturing process, the surface roughness can be made smaller by using abrasive particles with smaller diameters.
After the surface of a glass surface is polished as above, the polishing tape which has been running is removed from the substrate surface, the supply of the suspension with abrasive particles is stopped and a washing liquid such as water is applied to the substrate surface to clean it while the substrate is kept rotating. While the polishing tape is pressed and run and the glass substrate is kept rotating, a large amount of the suspension remains near the contact area between the polishing tape and the substrate. Thus, if the polishing tape were removed from the substrate surface after the rotary motion of the glass substrate and the movement of the polishing tape are stopped, a glass hydrate would be formed chemically as explained above on the contact boundary surface between the abrasive particles in the suspension remaining near the contact area between the polishing tape and the substrate surface, forming local protrusions of this glass hydrate on the surface of the glass substrate. Such protrusions would not be removable from the surface of the glass substrate by merely applying a washing liquid thereon. In other words, unwanted protrusions would be formed as a result. According to the present invention, therefore, the polishing tape is removed from the surface of the glass substrate, the supply of the suspension with abrasive particles is stopped and a washing liquid such as water is applied while the substrate is kept rotating such that the centrifugal force due to the rotary motion of the substrate can be taken advantage of to remove the abraded particles and the leftover suspension liquid still remaining on the surface of the substrate can be removed therefrom.
The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:
FIG. 1 is a schematic drawing of a polishing machine for using a method of polishing embodying this invention;
FIG. 2 is an enlarged photograph showing a plan view of a portion of a surface of a glass substrate polished in Test 1 described below;
FIG. 3 is an enlarged photograph showing a diagonal view of a portion of a surface of a glass substrate polished in Test 1 described below;
FIG. 4 is an enlarged photograph showing a plan view of a portion of a surface of a glass substrate polished in Test 2 described below;
FIG. 5 is an enlarged photograph showing a diagonal view of a portion of a surface of a glass substrate polished in Test 2 described below;
FIG. 6 is an enlarged photograph showing a plan view of a portion of a surface of a glass substrate polished in Test 3 described below; and
FIG. 7 is an enlarged photograph showing a diagonal view of a portion of a surface of a glass substrate polished in Test 3 described below.